1. Field of the Invention
This invention relates to cardiac pacemakers, and more particularly to pacemakers which combine sensor based, rate determining algorithms with atrial synchronized pacing in dual chamber rate responsive modes of physiologic pacing.
2. Description of the Prior Art
The commonly assigned U.S. Pat. No. 4,932,406 (incorporated herein by reference) sets forth the prior art of single and dual chamber pacing and physiologic sensor based rate responsive pacing. Dual chamber DDD physiologic pacemakers following the teachings of U.S. Pat. No. 4,312,355 to Funke may be characterized as possessing a number of different pacing modes which are switched in or out in the presence or absence of atrial and ventricular events. Such DDD pacemakers are constantly updating their functional modes at the end of programmed escape intervals or upon earlier occurring atrial and ventricular events. However, DDD pacemakers do not switch modes in the sense that that expression has been defined in the art.
"Mode switching" connotes a semi-permanent mode change driven by sensed heart activity events and/or sensor derived events occurring in a first relationship wherein the device dictates that it remain in the mode it is changed to until those or other events satisfy a second defined relationship. Individual "modes" define sets of rules for the operating states of the machine. In the pacing context, the rules define the pacing and sensing operating conditions from instant to instant irrespective of higher level monitoring of cardiac and physiologic signals which are used to initiate switching between modes, depending on a further set of mode switching rules. These "modes" include the major or principal operating modes and transition modes as described hereinafter.
For example, one of the earliest mode switching devices is illustrated by pacemakers exhibiting hysteresis and
particularly pacemakers of the type described in U.S. Pat. Nos. 4,363,325 to Roline et al and U.S. Pat. No. 3,999,557 to Citron. In the '325 patent, a pacemaker capable of operating in the atrial synchronous ventricular inhibited (VDD) mode automatically switches to the VVI mode at a preset atrial sense driven upper rate. The mode is switched back to VDD at a detected lower atrial sense driven lower rate. In the '557 patent, a VVI pacemaker remains "off" until a bout of extreme bradycardia (a heart rate less than 30 bpm) is detected, whereupon the VVI pacemaker switches "on" (at a lower rate of 70 bpm, for example). Thereafter the pacemaker remains in the VVI mode.
Contemporaneously with the introduction of dual chamber pacing, particularly DDD pacing, single chamber and subsequently dual chamber rate responsive pacemakers were developed as described in the aforementioned application. With the introduction and incorporation of physiologic sensors into single and dual chamber pacemakers, a four letter code denoting the principal modes of operation of pacemakers with and without physiologic sensors and rate adaptive pacing capabilities was published in "The NASPE/BPEG Pacemaker Code" by the A. Berstein al, PACE 10(4), Jul-Aug, 1987, which updated the three letter code published in the American Journal of Cardiology 34:487 (1974). The incorporation of physiologic sensors added impetus to the acceptance of the "mode switching" vernacular. Merely by adding a physiologic sensor, the pacemaker, regardless of whether it possessed only single chamber or dual chamber pacing capabilities, became susceptible to at least two modes of operation, that is, a first mode dictated by the heart rate, and a second mode dictated by the sensor derived rate. Thus, pacemakers with sensors for detecting physiologically required pacing rates (other than the underlying atrial and/or ventricular natural depolarization rates) switched modes between the sensor derived rate and the natural cardiac depolarization derived rate as taught, for example, in U.S. Pat. No. 4,890,617, incorporated herein by reference.
Consequently, the current vernacular for "mode switching" in the rate responsive pacing context suggests that the pacing mode of operation at any given time may be switched from a mode driven exclusively by the intrinsic heart rate during one period of time and the physiologic sensor derived heart rate during another period of time. In that context, the aforementioned '406 patent suggests a form of mode switching from the recognized DDD mode to the VVIR mode and back to the DDD mode under certain conditions related to the relationship between the natural atrial rate and a fallback limit rate. The solution to the problem of tracking the atrial rate in relation to a sensor derived rate proposed by the '406 patent encompasses switching modes between the DDD mode and the VVIR mode with specified transition mode and pacing rate rise-up and fallback behaviors.
Pacing in the VVIR mode may unnecessarily constrain the contribution of the atrial chamber of the heart in those situations when the physiologic sensor derived rate compared to the average atrial rate indicates that the atrium is chronatropically incompetent or that atrial undersensing is being experienced. It is generally desirable to maintain atrial contribution whenever possible and to maintain a certain smoothness in the stimulation rate between upper and lower rate limits.
It is also generally desirable to use the simplest modes possible to control the beat by beat operation of the system dependant upon two separate rate modifiers, that is, the instantaneous atrial rate in atrial synchronous modes and the average (combined) sensor driven rate in rate responsive modes. Moreover, it is desirable to maintain DDD pacing, that is, to track the atrial rate, whenever reasonable, and to switch modes only when accumulated evidence is available that the atrial rate is pathologic, noise, or incorrectly sensed. Reliance on the sensor driven rate should be minimized and limited only to situations where pathologic atrial tachycardia or atrial bradycardia are indicated given the inherent inferiority in specificity and reliability of man-made implanted sensors.
In addition, it is desirable that a smooth rate transition or a rate transition that is not felt by the patient be achieved. On the other hand, the system should not oscillate indecisively between modes or be too sensitive. In addition, pacing hemodynamics should not be compromised between or during transitions.